Ventilation towers are often incorporated into the design of naturally-ventilated buildings. These towers increase the physical height of the building and thereby potentially enhance the buoyancy-induced air velocity. However, acoustic baffles, insect meshes, etc., placed within the towers result in pressure losses that effectively reduce the area of the flow path, thereby restricting the rate of airflow. These competing effects (enhanced velocity and reduced area) mean that without careful attention to design, a building with a tower may yield lower airflow rates than a building without a tower. Industry has sought guidance on the effect of these towers on the ventilation of the adjoining building and we derive conditions for a ventilation tower to have a beneficial effect on the airflow rate. Conditions are developed for passive-stack ventilation driven by distributed and localised internal heat sources and then extended to take into account solar heat gains. Based on these conditions, design curves are presented together with a methodology for estimating the minimum tower height that will not be detrimental to the ventilation flow rate. An example is given illustrating the use of this technique.